Induction lamp



July 8, 1958 4 s, c, E JR 2,842,698

INDUCTION LAMP Filed March 30. 1956 v z I v1 5 /2 U 2 2 T 1 J 4 w /aFT'gLS JF1-4 IN V EN TOR. JA/VflFflRD CHR/JTOPHER PEEK, JR.

ATTORNEY HNDUCTION LAMP Sandford Christopher Peels, Jr., Hamilton, Mass,assignor to Sylvania Electric Products Inc, Salem, Mace, a corporationof Massachusetts Application March 30, 1956, Serial No, 575,1i2

1 Claim. (Cl. 313 161) This invention relates to induction lamps, i. e.,to lamps in which a refractory body, called a target, is inductivelyheated to incandescence.

Inductive heating is usually effected by placing a coil around therefractory body and passing a high frequency current through the coil. Afield concentrator is dis posed between the target and the coil, theconcentrator usually including an annular disc around the target butspaced from it. The disc has a radial slot to prevent circulation ofcurrents in a complete ring around the target. The target is usually acircular cylinder centrally located inside the central circular openingof the concentrator disc.

Although such an arrangement gives a far more uniform light distributionthan can be obtained from a coiled incandescent filament, the lightoutput will nonetheless be smaller from the target area in the vicinityof the slot than it will be in other regions of the target. Thebrightness is about 20% less near the slot than it is on thediametrically opposite portion of the target. Such a variation ispermissible for many purposes, but it is undesirable in the printing ofmotion picture film, for example.

This non-uniformity of brightness is of two general types. One issymmetrical non-uniformity due to the fact that most of the heatingoriginates near the outer edge; this tends to make the center of thedisc cooler than the outer edge. The second is asymmetricalnonuniformity which is caused by the presence of the slot in theconcentrator. The portion of the disc next to the slot tends to becooler than the portion remote from the slot. This latter kind ofnon-uniformity is undesirable for some applications such as some motionpicture printers. For some applications the symmetrical non-uniformityis actually desirable since it tends to compensate the tendency ofnormal optical systems to produce a hot spot in the middle of the imageof a uniform light source.

Diminution of asymmetrical non-uniformity may be obtained by narrowingthe slot in the concentrator. The narrower the slot the better, for asit is narrowed the current distribution in the concentrator tends to bemore symmetrical. However, the slot cannot be made so narrow that theradio frequency voltage causes an arc across the slot. Even though thenarrow slot helps, still further improvement is desirable.

In accordance with the principles of this invention, further diminutionof asymmetrical non-uniformity may be obtained by grinding the target sothat it is thinner on the side nearest the slot. This causes the sidenearest the slot to become hotter for two reasons; the thinnercrosssection produces a higher resistance to the induced cur-.

rents which in turn increases the heating, and the thinner Patented July8, 1958 cross-section conducts away less heat thus retaining more heaton the side cut away.

An induction lamp made in accordance with the principles of thisinvention is characterized by a symmetrical non-uniformity of less than10% and an asymmetrical non-uniformity of less than 5%, thus providing alamp very satisfactory for even the most critical applications.

In the accompanying drawings- Figure l is a transverse sectional view ofan induction lamp taken along the line l1 of Figure 2.

Figure 2 is a fragmentary sectional, elevational view on an enlargedscale of an induction lamp.

Figure 3 is a side elevational view on an enlarged scale of an inductionlamp target made in accordance with the principles of this invention.

Figure 4 is a bottom view of the induction lamp target of Figure 3.

An induction lamp of the type with which the target of this inventionmay be employed is shown and described in detail in the co-pendingapplication of S. C. Peek, In, Serial Number 520,718, filed July 8,1955. The lamp comprises a glass envelope 2 having a flat glass disc 4on an end thereof, through which light from a refractory target 6 mayemerge when the target is heated inductively, such as by the passage ofa high frequency current through a coil disposed about the envelope 2,for example. The target 6 is provided with a depending shank 8 whichfits into a support tube 10 of refractory material, such as zirconia forexample. The target 6 is disposed in a central aperture 12 formed in anannular metal disc 14 which constitutes the closed end of a cylindricalmetal shell 16 mounted in the envelope 2 on supports 18. The annularmetal disc 14 is provided with a narrow radial slot 20 to prevent theformation of a complete conductive turn in which current can flow byinduction.

In the specific embodiment of the invention illustrated in theaccompanying drawing, the annular metal disc 14 is about 2 inches indiameter and about inch thick. The slot 20 is about inch wide. The topsurface of the target 6 is about 4 inch in diameter. The side nearestthe slot 20 is ground down to about 5 inch thickness whereas the farside is about ,4; inch thick. The shank 8 tapers down to about & inch indiameter. Tantalum carbide, zirconium carbide, niobium carbide andhafnium carbide are examples of target materials which may be employedsatisfactorily.

Although specific dimensions of the specific embodiment of the targethave been given in the description of the specific embodiment of theinvention, it should be appreciated that variations may be made withinthe scope of the principles of the invention stated above.

What I claim is:

An induction lamp comprising a magnetic field concentrator having anarrow radial slot and a central opening formed therein, the radial slotextending from the References Cited in the file of this patent UNITEDSTATES PATENTS 2,774,905 Hanson Dec. 18, 1956

